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<p>Classes and structs are user-defined types, defined by class-specifier, which appears in <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration</a> syntax. The class specifier has the following syntax:
</p>
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<td class="t-sdsc-nopad"> <span class="t-spar">class-key</span> <span class="t-spar">attr</span> <span class="t-spar">class-head-name</span> <span class="t-spar">base-clause</span> <code><b>{ </b></code> <span class="t-spar">member-specification</span> <code><b>} </b></code>
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<td>  <span class="t-spar">class-key</span>
</td>
<td> -
</td>
<td>  one of <a href="../keyword/class.html" title="cpp/keyword/class">class</a> or <a href="../keyword/struct.html" title="cpp/keyword/struct">struct</a>.  The keywords are identical except for the default <a href="access.html" title="cpp/language/access">member access</a> and the default <a href="derived_class.html" title="cpp/language/derived class">base class access</a>.
</td>
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<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td>  optional sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>, may include <a href="alignas.html" title="cpp/language/alignas">alignas specifier</a>
</td>
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<td>  <span class="t-spar">class-head-name</span>
</td>
<td> -
</td>
<td>  the name of the class that's being defined. Optionally <a href="name.html#Qualified_identifiers" title="cpp/language/identifiers">qualified</a>, optionally followed by keyword <a href="../keyword/final.html" title="cpp/keyword/final">final</a>. The name may be omitted, in which case the class is unnamed (note that unnamed class cannot be <a href="final.html" title="cpp/language/final">final</a>)
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<td>  <span class="t-spar">base-clause</span>
</td>
<td> -
</td>
<td>  optional list of one or more parent classes and the model of inheritance used for each (see <a href="derived_class.html" title="cpp/language/derived class">derived class</a>)
</td>
</tr>
<tr class="t-par">
<td>  <span class="t-spar">member-specification</span>
</td>
<td> -
</td>
<td>  list of access specifiers, member object and member function declarations and definitions (see below)
</td>
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<p>See <a href="classes.html" title="cpp/language/classes">classes</a> for general overview of the syntax. If <span class="t-spar">class-key</span> is <code>union</code>, the declaration introduces a <a href="union.html" title="cpp/language/union">union type</a>. 
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="class.html#Forward_declaration"><span class="tocnumber">1</span> <span class="toctext">Forward declaration</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="class.html#Member_specification"><span class="tocnumber">2</span> <span class="toctext">Member specification</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="class.html#Local_classes"><span class="tocnumber">3</span> <span class="toctext">Local classes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="class.html#See_also"><span class="tocnumber">4</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Forward declaration">edit</a>]</span> <span class="mw-headline" id="Forward_declaration">Forward declaration</span>
</h3>
<p>A declaration of the following form
</p>
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<td colspan="10" class="t-sdsc-sep">
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<td class="t-sdsc-nopad"> <span class="t-spar">class-key</span> <span class="t-spar">attr</span> <span class="t-spar">identifier</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td>
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<p>Declares a class type which will be defined later in this scope. Until the definition appears, this class name has <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete type</a>. This allows classes that refer to each other:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Vector<span class="sy4">;</span> <span class="co1">// forward declaration</span>
<span class="kw1">class</span> Matrix <span class="br0">{</span>
    <span class="co1">// ...</span>
    <span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&amp;</span>, <span class="kw4">const</span> Vector<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Vector <span class="br0">{</span>
    <span class="co1">// ...</span>
    <span class="kw1">friend</span> Vector operator<span class="sy2">*</span><span class="br0">(</span><span class="kw4">const</span> Matrix<span class="sy3">&amp;</span>, <span class="kw4">const</span> Vector<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>and if a particular source file only uses pointers and references to the class, this makes it possible to reduce #include dependencies:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// in MyStruct.h </span>
<span class="co2">#include &lt;iosfwd&gt; // contains forward declaration of std::ostream</span>
<span class="kw1">struct</span> MyStruct <span class="br0">{</span>
    <span class="kw4">int</span> value<span class="sy4">;</span>
    <span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1417">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> operator<span class="sy1">&lt;&lt;</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1417">std::<span class="me2">ostream</span></span></a><span class="sy3">&amp;</span> os, <span class="kw4">const</span> S<span class="sy3">&amp;</span> s<span class="br0">)</span><span class="sy4">;</span>
    <span class="co1">// definition provided in MyStruct.cpp file which uses #include &lt;ostream&gt;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If forward declaration appears in local scope, it <i>hides</i> previously declared class, variable, function, and all other declarations of the same name that may appear in enclosing scopes:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// does nothing (s already defined in this scope)</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw1">struct</span> s<span class="sy4">;</span> <span class="co1">// forward declaration of a new, local struct "s"</span>
              <span class="co1">// this hides global struct s until the end of this block</span>
    s<span class="sy2">*</span> p<span class="sy4">;</span>     <span class="co1">// pointer to local struct s</span>
    <span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">char</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definitions of the local struct s</span>
<span class="br0">}</span></pre></div></div>
<p>Note that a new class name may also be introduced by an <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier">elaborated type specifier</a> which appears as part of another declaration, but only if <a href="lookup.html" title="cpp/language/lookup">name lookup</a> can't find a previously declared class with the same name.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> U<span class="sy4">;</span>
<span class="kw1">namespace</span> ns<span class="br0">{</span>
    <span class="kw1">class</span> Y f<span class="br0">(</span><span class="kw1">class</span> T p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares function ns::f and declares ns::T and ns::Y</span>
    <span class="kw1">class</span> U f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// U refers to ::U</span>
    Y<span class="sy2">*</span> p<span class="sy4">;</span> T<span class="sy2">*</span> q<span class="sy4">;</span> <span class="co1">// can use pointers and references to T and Y</span>
<span class="br0">}</span></pre></div></div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Member specification">edit</a>]</span> <span class="mw-headline" id="Member_specification">Member specification</span>
</h3>
<p>The member specification, or the <i>body</i> of a class definition, is a brace-enclosed sequence of any number of the following:
</p>
<div class="t-li1">
<span class="t-li">1)</span> Member declarations, which are <a href="declarations.html" title="cpp/language/declarations">declarations</a> of <a href="static.html" title="cpp/language/static">static</a> and non-static <a href="data_members.html" title="cpp/language/data members">data members</a> and <a href="member_functions.html" title="cpp/language/member functions">member functions</a>, <a href="typedef.html" title="cpp/language/typedef">typedef</a> declarations, scoped and unscoped <a href="enum.html" title="cpp/language/enum">enumerations</a>, and other, <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested</a>, class declarations. 
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> S <span class="br0">{</span>
    <span class="kw4">int</span> d1<span class="sy4">;</span> <span class="co1">// non-static data member</span>
    <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// non-static data member with initializer (C++11)</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> d2 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// static data member with initializer</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// pure virtual member function</span>
    <a href="../string/basic_string.html"><span class="kw1042">std::<span class="me2">string</span></span></a> d3, <span class="sy2">*</span>d4, f2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// two data members and a member function</span>
    <span class="kw2">enum</span> <span class="br0">{</span>NORTH, SOUTH, EAST, WEST<span class="br0">}</span><span class="sy4">;</span>
    <span class="kw1">struct</span> NestedS <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1042">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
    <span class="br0">}</span> d5, <span class="sy2">*</span>d6<span class="sy4">;</span>
    <span class="kw1">typedef</span> NestedS value_type, <span class="sy2">*</span>pointer_type<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">2)</span> Function definitions, which both declare and define <a href="member_functions.html" title="cpp/language/member functions">member functions</a>. A semicolon after a member function definition is optional. All member functions that are defined inside a class body are automaticaly <a href="inline.html" title="cpp/language/inline">inline</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> M <span class="br0">{</span>
    <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> C<span class="sy4">;</span>
    <a href="../container/vector.html"><span class="kw1064">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> data<span class="sy4">;</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    M<span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> R, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> C<span class="br0">)</span> <span class="sy4">:</span> C<span class="br0">(</span>C<span class="br0">)</span>, data<span class="br0">(</span>R<span class="sy2">*</span>C<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// constructor definition </span>
    <span class="kw4">int</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="co1">// member function definition</span>
        <span class="kw1">return</span> data<span class="br0">[</span>r<span class="sy2">*</span>C<span class="sy2">+</span>c<span class="br0">]</span><span class="sy4">;</span>
    <span class="br0">}</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>size_t r, size_t c<span class="br0">)</span> <span class="br0">{</span>  <span class="co1">// another member function definition</span>
        <span class="kw1">return</span> data<span class="br0">[</span>r<span class="sy2">*</span>C<span class="sy2">+</span>c<span class="br0">]</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">3)</span> <a href="access.html" title="cpp/language/access">Access specifiers</a> <code>public:</code>, <code>protected:</code>, and <code>private:</code>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> S <span class="br0">{</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>          <span class="co1">// public constructor</span>
    S<span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// public copy constructor</span>
    <span class="kw1">virtual</span> ~S<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// public virtual destructor</span>
 <span class="kw1">private</span><span class="sy4">:</span>
    <span class="kw4">int</span><span class="sy2">*</span> ptr<span class="sy4">;</span> <span class="co1">// private data member</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">4)</span> <a href="using_declaration.html" title="cpp/language/using declaration">Using-declarations</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Base <span class="br0">{</span>
 <span class="kw1">protected</span><span class="sy4">:</span>
     <span class="kw4">int</span> d<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Derived <span class="sy4">:</span> <span class="kw1">public</span> Base <span class="br0">{</span>
 <span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">d</span><span class="sy4">;</span> <span class="co1">// make Base's protected member d a public member of Derived</span>
    <span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">Base</span><span class="sy4">;</span> <span class="co1">// inherit all parent's constructors (C++11)</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">5)</span> <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declarations
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> Foo <span class="br0">{</span>
  static_assert<span class="br0">(</span><a href="../types/is_floating_point.html"><span class="kw443">std::<span class="me2">is_floating_point</span></span></a><span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>, <span class="st0">"Foo&lt;T&gt;: T must be floating point"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">6)</span> <a href="member_template.html" title="cpp/language/member template">member template declarations</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T<span class="sy3">&amp;&amp;</span> n<span class="br0">)</span><span class="sy4">;</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> CharT<span class="sy1">&gt;</span>
    <span class="kw1">struct</span> NestedS <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1040">std::<span class="me2">basic_string</span></span></a><span class="sy1">&lt;</span>CharT<span class="sy1">&gt;</span> s<span class="sy4">;</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">7)</span> <a href="type_alias.html" title="cpp/language/type alias">alias declarations</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> identity
<span class="br0">{</span>
    <span class="kw1">using</span> type <span class="sy1">=</span> T<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Local classes">edit</a>]</span> <span class="mw-headline" id="Local_classes">Local classes</span>
</h3>
<p>A class declaration can appear in namespace scope (in which case it defines an ordinary class), inside another class definition (in which case it defines a <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested class</a>), and inside the body of a function, in which case it defines a <i>local class</i>. This class only exists within the function scope, and is not accessible outside.
</p>
<ul>
<li> A local class cannot have static members
</li>
<li> Member functions of a local class have no linkage
</li>
<li> Member functions of a local class have to be defined entirely inside the class body
</li>
<li> Local classes cannot have member templates or friend templates
</li>
<li> A local class inside a function (including member function) can access the same names that the enclosing function can access.
</li>
</ul>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx11">
<td>
<ul><li> local classes could not be used as template arguments
</li></ul>
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td>
</tr>
</table>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;algorithm&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../container/vector.html"><span class="kw1064">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> v<span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="kw1">struct</span> Local <span class="br0">{</span>
       <span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n, <span class="kw4">int</span> m<span class="br0">)</span> <span class="br0">{</span>
           <span class="kw1">return</span> n <span class="sy1">&gt;</span> m<span class="sy4">;</span>
       <span class="br0">}</span>
    <span class="br0">}</span><span class="sy4">;</span>
    <a href="../algorithm/sort.html"><span class="kw1316">std::<span class="me2">sort</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Local<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// since C++11</span>
    <span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n<span class="sy4">:</span> v<span class="br0">)</span> <a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">' '</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">3 2 1</pre></div></div> 
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see">
<span><a href="../../c/language/struct.html" title="c/language/struct">C documentation</a></span> for <span class="t-dsc-see-tt"><span>Struct declaration</span></span>
</div>
</td>
</tr>

</table>

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